Moisture is known to be a critical contributing factor in sustaining the degradation mechanisms in concrete - most widespread being the corrosion of embedded steel reinforcement. So, whether providing a cover depth corresponding the estimated maximum moisture penetration depth, under the given conditions of service, an effective method to keep the reinforcement steel immune from corrosion ?. Please share your thoughts.
Building regulations of most countries require the cover depth in terms of carbonation depth that may reach the position of reinforcing steel within the designed service lives of concrete structures. In addition to this basic design requirement, control of the moisture conditions, e.g. using water-repellent surface treatment, to enhance the designed service life is a good option. The maximum moisture penetration needs to be explained in detail.
Concrete cover acts as a fire barrier for the reinforcements. It also has some effects on the required development and anchorage lengths of the rebar. In unconfined regions (column mid-height or upper story shear walls) with thin cover, rebars under compression would be buckle prone.
Obviously the story of concrete cover is not only about corrosion concerns.
it is the porosity of of the concrete matters not the thickness
i found out the porosity of the concrete in 1968 by weight loss methods
"pore space in concrete " - the pores retain the water and also chlorides and sulphates
surface coating on concrete is good as a corrosion prevention method
Dear Seyed. Thank you for sharing your thoughts. Agree to you.
Of course one will have to adopt the max value to satiate all possible aspects.
My question is specific to durability requirements.
For durability remind that the steel reveal is protected because the Ph is high and the presence of Carbonation and or Chlorides decreases this value to a critical one; about 7 tu 8 depending on the chemical composition of the concrete. The more the concrete can preserve a basic environment the more the protection of steel rebar is effective,
Dear Prof.Muralidharan. Thank you sir for sharing your ideas.
Connected Porosity of concrete is indeed a critical aspect. And then there will be variations in the porosity of surface zone and the core of concrete. Modelling of porosity (open/connected) is still an open area of research.
However, say if we know the porosity beforehand, then, is it not more rational to have an estimate of probable limiting moisture penetration to decide for the cover requirements rather than relying on surface treatments. The presently recommended values of nominal cover in IS 456: 2000 are prescriptive and do not account for porosity or desired resistance against degradation.
Application of surface coating will definitely provide resistance against transport processes. But then, are there any reliable insights (except the commercial claims) to judge by what extent? Rather one may ask, what should be cover thickness to be provided when a surface treatment is to be given and what if it is not, for a given expected service life.
depends on the need - I know they give epoxy coal tar coat for bridges
Thank you, Mr.Tada for your response.
Any rational insights on the extent by which the "maintenance free service life" can be enhanced with surface coatings.
By "maximum moisture penetration", I mean the maximum depth up to which moisture can ingress in an exposed concrete medium subjected to the probable extremes of service environment. In fact the carbonation front will never penetrate beyond this depth.
Thank you, Mr.Franck.
Does the presence of chlorides decrease pH ?
Any specific insights on what the cover depth should be, under the given conditions of exposure, to enable concrete surrounding the rebar to remain alkaline.
Will it not be the depth up to which moisture just fails to penetrate ?
only carbonate decrease the pH --CHLORIDES cause pitting - breakdown of passivation
higher the cover carbonation will be slower - there is enough water in the pores while casting -
I imagine you may consider including the type of reinforcement as some types of reinforcement can resist corrosion like fiber reinforced polymer bars so the cover size will be mainly a function of the fire resistance. Considering the types of moisture exposure, since the exposure to aggressive environment needs more concern compared to as moisture in non aggressive environment. The maximum crack width associated with the design of reinforced concrete structure must be a controlling factor.
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